Golf ball

ABSTRACT

A golf ball  2  includes a core  4 , a mid layer  6 , and a cover  8 . A difference DH in hardness between a surface and a central point of the core  4 ; a thickness Tm (mm), a hardness Hm, and a volume Vm (mm 3 ) of the mid layer  6 ; and a thickness Tc (mm), a hardness Hc, and a virtual volume Vc (mm 3 ) of the cover  8  meet the following mathematical formulas. 
       ( DH*Hm )/( Hc*Tc )&gt;63 
       3.0&lt;(( Vm*Hm )/( Vc*Hc ))&lt;7.5 
     A dimple pattern of each hemisphere of the golf ball  2  includes three units that are rotationally symmetrical to each other. A dimple pattern of each unit includes two small units that are mirror-symmetrical to each other.

This application claims priority on Patent Application No. 2015-242977 filed in JAPAN on Dec. 14, 2015. The entire contents of this Japanese Patent Application are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to golf balls. Specifically, the present invention relates to golf balls including a core, a mid layer, a cover, and dimples.

Description of the Related Art

The greatest interest to golf players concerning golf balls is flight distance. In particular, golf players place importance on flight distances upon shots with a driver. There have been various proposals for improvement of flight performance. JP2010-188199 discloses a golf ball including a core having a high hardness at the surface thereof and a low hardness at the central point thereof. JP2013-31778 discloses a golf ball of which the hardness gradually increases from the central point of a core toward the surface of the core.

Golf balls have a large number of dimples on the surfaces thereof. The dimples disturb the air flow around the golf ball during flight to cause turbulent flow separation. This phenomenon is referred to as “turbulization”. Due to the turbulization, separation points of the air from the golf ball shift backwards leading to a reduction of drag. The turbulization promotes the displacement between the separation point on the upper side and the separation point on the lower side of the golf ball, which results from the backspin, thereby enhancing the lift force that acts upon the golf ball. Excellent dimples efficiently disturb the air flow. The excellent dimples produce a long flight distance.

There have been various proposals for dimples. JP2009-172192 (US2009/0191982) discloses a golf ball on which dimples are randomly arranged. The dimple pattern of the golf ball is referred to as a random pattern. The random pattern can contribute to the flight performance of the golf ball. JP2012-10822 (US2012/0004053) also discloses a golf ball having a random pattern.

JP2007-175267 (US2007/0149321) discloses a dimple pattern in which the number of units in a high-latitude region is different from the number of units in a low-latitude region. JP2007-195591 (US2007/0173354) discloses a dimple pattern in which the number of the types of dimples in a low-latitude region is larger than the number of the types of dimples in a high-latitude region. JP2013-153966 (US2013/0196791) discloses a dimple pattern having a high dimple density and small variation in dimple size.

Golf players also place importance on spin performance of golf balls. When a backspin rate is high, the run is short. By using a golf ball having a high backspin rate, a golf player can cause the golf ball to stop at a target point. When a sidespin rate is high, the golf ball tends to curve. By using a golf ball having a high sidespin rate, a golf player can intentionally cause the golf ball to curve. Golf balls having excellent spin performance have excellent controllability. Advanced golf players place importance particularly on controllability upon shots with a short iron.

Golf players' requirements for golf balls have been escalated. There is room for improvement in various performance characteristics of golf balls.

An object of the present invention is to provide a golf ball having excellent flight performance upon hitting with a driver and having excellent controllability upon hitting with a short iron.

SUMMARY OF THE INVENTION

A golf ball according to the present invention includes a core, a mid layer positioned outside the core, and a cover positioned outside the mid layer. A difference DH in Shore C hardness between a surface and a central point of the core; a thickness Tm (mm), a Shore D hardness Hm, and a volume Vm (mm³) of the mid layer; and a thickness Tc (mm), a Shore D hardness Hc, and a virtual volume Vc (mm³) of the cover meet the following mathematical formulas (1.1) and (1.2).

(DH*Hm)/(Hc*Tc)>63  (1.1)

3.0<((Vm*Hm)/(Vc*Hc))<7.5  (1.2)

The hardness Hc of the cover is equal to or less than 40. The golf ball has a plurality of dimples on a surface thereof. A ratio So of a sum of areas of the dimples relative to a surface area of a phantom sphere of the golf ball is equal to or greater than 81.0%. A ratio Rs of a number of the dimples each having a diameter of equal to or greater than 9.60% but equal to or less than 10.37%, of a diameter of the golf ball, relative to a total number of the dimples, is equal to or greater than 50%. A dimple pattern of each hemisphere of the golf ball includes three units that are rotationally symmetrical to each other. A dimple pattern of each unit includes two small units that are mirror-symmetrical to each other. The golf ball meets the following mathematical formula (2.1).

Rs≧−2.5*So+273  (2.1)

When the golf ball according to the present invention is hit with a driver, the spin rate is low. Furthermore, the dimple pattern of the golf ball has an excellent aerodynamic characteristic. The golf ball has excellent flight performance upon hitting with a driver. When the golf ball is hit with a short iron, the spin rate is high. The golf ball has excellent controllability upon hitting with a short iron. The golf ball is excellent in both flight performance upon hitting with a driver and controllability upon hitting with a short iron.

Preferably, the hardness Hm of the mid layer is equal to or greater than 55.

Preferably, the golf ball meets the following mathematical formula (2.2).

Rs≧−2.5*So+278  (2.2)

Preferably, the golf ball meets the following mathematical formula (2.3).

Rs≧−2.5*So+283  (2.3)

Preferably, a ratio Rs′ of a number of the dimples each having a diameter of equal to or greater than 10.10% but equal to or less than 10.37%, of the diameter of the golf ball, relative to the total number of the dimples, is equal to or greater than 50%. Preferably, the golf ball meets the following mathematical formula (2.4).

Rs′≧−2.2*So+245  (2.4)

Preferably, the golf ball meets the following mathematical formula (2.5).

Rs′≧−2.2*So+252  (2.5)

Preferably, a depth of a deepest part of each dimple from a surface of the phantom sphere is equal to or greater than 0.10 mm but equal to or less than 0.65 mm.

Preferably, a total volume of the dimples is equal to or greater than 450 mm³ but equal to or less than 750 mm³.

Preferably, a ratio of a number of the dimples each having a diameter exceeding 10.37% of the diameter of the golf ball, relative to the total number of the dimples, is less than 50%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a golf ball according to one embodiment of the present invention;

FIG. 2 is an enlarged plan view of the golf ball in FIG. 1;

FIG. 3 is a front view of the golf ball in FIG. 2;

FIG. 4 is a partially enlarged cross-sectional view of the golf ball in FIG. 1;

FIG. 5 is a graph showing a relationship between a ratio So and a ratio Rs;

FIG. 6 is a graph showing a relationship between the ratio So and a ratio Rs′;

FIG. 7 is a plan view of a golf ball according to Example 2 of the present invention;

FIG. 8 is a front view of the golf ball in FIG. 7;

FIG. 9 is a plan view of a golf ball according to Comparative Example 1; and

FIG. 10 is a front view of the golf ball in FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe in detail the present invention based on preferred embodiments with appropriate reference to the drawings.

A golf ball 2 shown in FIG. 1 includes a spherical core 4, a mid layer 6 positioned outside the core 4, and a cover 8 positioned outside the mid layer 6. The golf ball 2 has a plurality of dimples 10 on the surface thereof. Of the surface of the golf ball 2, a part other than the dimples 10 is a land 12. The golf ball 2 includes a paint layer and a mark layer on the external side of the cover 8 although these layers are not shown in the drawing. The golf ball 2 may include another layer between the core 4 and the mid layer 6. The golf ball 2 may include another layer between the mid layer 6 and the cover 8.

The golf ball 2 preferably has a diameter of equal to or greater than 40 mm but equal to or less than 45 mm. From the standpoint of conformity to the rules established by the United States Golf Association (USGA), the diameter is particularly preferably equal to or greater than 42.67 mm. In light of suppression of air resistance, the diameter is more preferably equal to or less than 44 mm and particularly preferably equal to or less than 42.80 mm. The diameter of the golf ball 2 according to the present embodiment is 42.70 mm. Thirty points on the land 12 are selected at random. A diameter at each of the points as an end is measured. The diameter of the golf ball 2 is calculated by averaging these diameters.

The golf ball 2 preferably has a weight of equal to or greater than 40 g but equal to or less than 50 g. In light of attainment of great inertia, the weight is more preferably equal to or greater than 44 g and particularly preferably equal to or greater than 45.00 g. From the standpoint of conformity to the rules established by the USGA, the weight is particularly preferably equal to or less than 45.93 g.

The core 4 is formed by crosslinking a rubber composition. Examples of preferable base rubbers for use in the rubber composition include polybutadienes, polyisoprenes, styrene-butadiene copolymers, ethylene-propylene-diene copolymers, and natural rubbers. In light of resilience performance, polybutadienes are preferable. When a polybutadiene and another rubber are used in combination, it is preferred if the polybutadiene is a principal component. Specifically, the proportion of the polybutadiene to the entire base rubber is preferably equal to or greater than 50% by weight and particularly preferably equal to or greater than 80% by weight. A polybutadiene in which the proportion of cis-1,4 bonds is equal to or greater than 80% is particularly preferable.

The rubber composition of the core 4 preferably includes a co-crosslinking agent. Preferable co-crosslinking agents in light of resilience performance are monovalent or bivalent metal salts of an α,β-unsaturated carboxylic acid having 2 to 8 carbon atoms. Examples of preferable co-crosslinking agents include zinc acrylate, magnesium acrylate, zinc methacrylate, and magnesium methacrylate. In light of resilience performance, zinc acrylate and zinc methacrylate are particularly preferable.

The rubber composition may include a metal oxide and an α,β-unsaturated carboxylic acid having 2 to 8 carbon atoms. They both react with each other in the rubber composition to obtain a salt. The salt serves as a co-crosslinking agent. Examples of preferable α,β-unsaturated carboxylic acids include acrylic acid and methacrylic acid. Examples of preferable metal oxides include zinc oxide and magnesium oxide.

In light of resilience performance of the golf ball 2, the amount of the co-crosslinking agent per 100 parts by weight of the base rubber is preferably equal to or greater than 10 parts by weight and particularly preferably equal to or greater than 15 parts by weight. In light of feel at impact, the amount is preferably equal to or less than 50 parts by weight and particularly preferably equal to or less than 45 parts by weight.

Preferably, the rubber composition of the core 4 includes an organic peroxide. The organic peroxide serves as a crosslinking initiator. The organic peroxide contributes to the resilience performance of the golf ball 2. Examples of suitable organic peroxides include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. An organic peroxide with particularly high versatility is dicumyl peroxide.

In light of resilience performance of the golf ball 2, the amount of the organic peroxide per 100 parts by weight of the base rubber is preferably equal to or greater than 0.1 parts by weight, more preferably equal to or greater than 0.3 parts by weight, and particularly preferably equal to or greater than 0.5 parts by weight. In light of feel at impact, the amount is preferably equal to or less than 3.0 parts by weight, more preferably equal to or less than 2.8 parts by weight, and particularly preferably equal to or less than 2.5 parts by weight.

Preferably, the rubber composition of the core 4 includes an organic sulfur compound. Organic sulfur compounds include naphthalenethiol compounds, benzenethiol compounds, and disulfide compounds.

Examples of naphthalenethiol compounds include 1-naphthalenethiol, 2-naphthalenethiol, 4-chloro-1-naphthalenethiol, 4-bromo-1-naphthalenethiol, 1-chloro-2-naphthalenethiol, 1-bromo-2-naphthalenethiol, 1-fluoro-2-naphthalenethiol, 1-cyano-2-naphthalenethiol, and 1-acetyl-2-naphthalenethiol.

Examples of benzenethiol compounds include benzenethiol, 4-chlorobenzenethiol, 3-chlorobenzenethiol, 4-bromobenzenethiol, 3-bromobenzenethiol, 4-fluorobenzenethiol, 4-iodobenzenethiol, 2,5-dichlorobenzenethiol, 3,5-dichlorobenzenethiol, 2,6-dichlorobenzenethiol, 2,5-dibromobenzenethiol, 3,5-dibromobenzenethiol, 2-chloro-5-bromobenzenethiol, 2,4,6-trichlorobenzenethiol, 2,3,4,5,6-pentachlorobenzenethiol, 2,3,4,5,6-pentafluorobenzenethiol, 4-cyanobenzenethiol, 2-cyanobenzenethiol, 4-nitrobenzenethiol, and 2-nitrobenzenethiol.

Examples of disulfide compounds include diphenyl disulfide, bis(4-chlorophenyl)disulfide, bis(3-chlorophenyl)disulfide, bis(4-bromophenyl)disulfide, bis(3-bromophenyl)disulfide, bis(4-fluorophenyl)disulfide, bis(4-iodophenyl)disulfide, bis(4-cyanophenyl)disulfide, bis(2,5-dichlorophenyl)disulfide, bis(3,5-dichlorophenyl)disulfide, bis(2,6-dichlorophenyl)disulfide, bis(2,5-dibromophenyl)disulfide, bis(3,5-dibromophenyl)disulfide, bis(2-chloro-5-bromophenyl)disulfide, bis(2-cyano-5-bromophenyl)disulfide, bis(2,4,6-trichlorophenyl)disulfide, bis(2-cyano-4-chloro-6-bromophenyl)disulfide, bis(2,3,5,6-tetrachlorophenyl)disulfide, bis(2,3,4,5,6-pentachlorophenyl)disulfide, and bis(2,3,4,5,6-pentabromophenyl)disulfide.

In light of resilience performance of the golf ball 2, the amount of the organic sulfur compound per 100 parts by weight of the base rubber is preferably equal to or greater than 0.1 parts by weight and particularly preferably equal to or greater than 0.2 parts by weight. In light of feel at impact, the amount is preferably equal to or less than 1.5 parts by weight, more preferably equal to or less than 1.0 parts by weight, and particularly preferably equal to or less than 0.8 parts by weight. Two or more organic sulfur compounds may be used in combination. A naphthalenethiol compound and a disulfide compound are preferably used in combination.

Preferably, the rubber composition of the core 4 includes a carboxylic acid or a carboxylate. The core 4 including a carboxylic acid or a carboxylate has a low hardness around the central point thereof. The core 4 has an outer-hard/inner-soft structure. Examples of preferable carboxylic acids include benzoic acid. Examples of preferable carboxylates include zinc octoate and zinc stearate. The rubber composition particularly preferably includes benzoic acid. The amount of the carboxylic acid and/or the carboxylate per 100 parts by weight of the base rubber is preferably equal to or greater than 1 parts by weight but equal to or less than 20 parts by weight.

The rubber composition of the core 4 may include a filler for the purpose of specific gravity adjustment and the like. Examples of suitable fillers include zinc oxide, barium sulfate, calcium carbonate, and magnesium carbonate. The amount of the filler is determined as appropriate so that the intended specific gravity of the core 4 is accomplished. The rubber composition may include various additives, such as sulfur, an anti-aging agent, a coloring agent, a plasticizer, a dispersant, and the like, in an adequate amount. The rubber composition may include crosslinked rubber powder or synthetic resin powder.

The core 4 preferably has a diameter of equal to or greater than 38.0 mm. The golf ball 2 that includes the core 4 having a diameter of equal to or greater than 38.0 mm has excellent resilience performance. In this respect, the diameter is more preferably equal to or greater than 38.5 mm and particularly preferably equal to or greater than 39.5 mm. From the standpoint that the mid layer 6 and the cover 8 can have sufficient thicknesses, the diameter is preferably equal to or less than 41.0 mm and particularly preferably equal to or less than 40.5 mm.

The core 4 has a weight of preferably equal to or greater than 10 g but equal to or less than 40 g. The temperature for crosslinking the core 4 is equal to or higher than 140° C. but equal to or lower than 180° C. The time period for crosslinking the core 4 is equal to or longer than 10 minutes but equal to or shorter than 60 minutes. The core 4 may have two or more layers. The core 4 may have a rib on the surface thereof. The core 4 may be hollow.

In the golf ball 2, the difference DH between a hardness H1 at the central point of the core 4 and a hardness H2 at the surface of the core 4 is great. The core 4 having the great difference DH has a so-called outer-hard/inner-soft structure. When the golf ball 2 including the core 4 is hit with a driver, the spin is suppressed. When the golf ball 2 including the core 4 is hit with a driver, a high launch angle is obtained.

Upon a shot with a driver, an appropriate trajectory height and appropriate flight duration are required. With the golf ball 2 that achieves a desired trajectory height and desired flight duration at a high spin rate, the run after landing is short. With the golf ball 2 that achieves a desired trajectory height and desired flight duration at a high launch angle, the run after landing is long. In light of flight distance, the golf ball 2 that achieves a desired trajectory height and desired flight duration at a high launch angle is preferable. The core 4 having an outer-hard/inner-soft structure can contribute to a high launch angle and a low spin rate as described above. The golf ball 2 including the core 4 has excellent flight performance.

In light of spin suppression upon a shot with a driver, the difference DH is preferably equal to or greater than 15, more preferably equal to or greater than 18, and particularly preferably equal to or greater than 20. In light of ease of producing the core 4, the difference DH is preferably equal to or less than 50, more preferably equal to or less than 45, and particularly preferably equal to or less than 40.

In light of resilience performance, the central hardness H1 is preferably equal to or greater than 30, more preferably equal to or greater than 35, and particularly preferably equal to or greater than 40. In light of spin suppression upon a shot with a driver, the hardness H1 is preferably equal to or less than 70, more preferably equal to or less than 65, and particularly preferably equal to or less than 60.

The hardness H1 is measured with a Shore C type hardness scale mounted to an automated hardness meter (trade name “digi test II” manufactured by Heinrich Bareiss Prüfgeratebau GmbH). The hardness scale is pressed against the central point of the cross-section of a hemisphere obtained by cutting the golf ball 2. The measurement is conducted in the environment of 23° C.

In light of spin suppression upon a shot with a driver, the surface hardness H2 is preferably equal to or greater than 70, more preferably equal to or greater than 72, and particularly preferably equal to or greater than 74. In light of durability of the golf ball 2, the hardness H2 is preferably equal to or less than 90, more preferably equal to or less than 88, and particularly preferably equal to or less than 86.

The hardness H2 is measured with a Shore C type hardness scale mounted to an automated hardness meter (trade name “digi test II” manufactured by Heinrich Bareiss Prüfgeratebau GmbH). The hardness scale is pressed against the surface of the core 4. The measurement is conducted in the environment of 23° C.

The mid layer 6 is positioned between the core 4 and the cover 8. The mid layer 6 is formed from a resin composition. Examples of the base polymer of the resin composition include ionomer resins, thermoplastic polyester elastomers, thermoplastic polyamide elastomers, thermoplastic polyurethane elastomers, thermoplastic polyolefin elastomers, and thermoplastic polystyrene elastomers. Ionomer resins are particularly preferable. Ionomer resins are highly elastic. The golf ball 2 that includes the mid layer 6 including an ionomer resin has excellent resilience performance.

An ionomer resin and another resin may be used in combination. In this case, in light of resilience performance, the ionomer resin is included as the principal component of the base polymer. The proportion of the ionomer resin to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 70% by weight, and particularly preferably equal to or greater than 85% by weight.

Examples of preferable ionomer resins include binary copolymers formed with an α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. A preferable binary copolymer includes 80% by weight or more but 90% by weight or less of an α-olefin, and 10% by weight or more but 20% by weight or less of an α,β-unsaturated carboxylic acid. The binary copolymer has excellent resilience performance. Examples of other preferable ionomer resins include ternary copolymers formed with: an α-olefin; an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and an α,β-unsaturated carboxylate ester having 2 to 22 carbon atoms. A preferable ternary copolymer includes 70% by weight or more but 85% by weight or less of an α-olefin, 5% by weight or more but 30% by weight or less of an α,β-unsaturated carboxylic acid, and 1% by weight or more but 25% by weight or less of an α,β-unsaturated carboxylate ester. The ternary copolymer has excellent resilience performance. For the binary copolymer and the ternary copolymer, preferable α-olefins are ethylene and propylene, while preferable α,β-unsaturated carboxylic acids are acrylic acid and methacrylic acid. A particularly preferable ionomer resin is a copolymer formed with ethylene and acrylic acid. Another particularly preferable ionomer resin is a copolymer formed with ethylene and methacrylic acid.

In the binary copolymer and the ternary copolymer, some of the carboxyl groups are neutralized with metal ions. Examples of metal ions for use in neutralization include sodium ion, potassium ion, lithium ion, zinc ion, calcium ion, magnesium ion, aluminum ion, and neodymium ion. The neutralization may be carried out with two or more types of metal ions. Particularly suitable metal ions in light of resilience performance and durability of the golf ball 2 are sodium ion, zinc ion, lithium ion, and magnesium ion.

Specific examples of ionomer resins include trade names “Himilan 1555”, “Himilan 1557”, “Himilan 1605”, “Himilan 1706”, “Himilan 1707”, “Himilan 1856”, “Himilan 1855”, “Himilan AM7311”, “Himilan AM7315”, “Himilan AM7317”, “Himilan AM7329”, and “Himilan AM7337”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.; trade names “Surlyn 6120”, “Surlyn 6910”, “Surlyn 7930”, “Surlyn 7940”, “Surlyn 8140”, “Surlyn 8150”, “Surlyn 8940”, “Surlyn 8945”, “Surlyn 9120”, “Surlyn 9150”, “Surlyn 9910”, “Surlyn 9945”, “Surlyn AD8546”, “HPF1000”, and “HPF2000”, manufactured by E.I. du Pont de Nemours and Company; and trade names “IOTEK 7010”, “IOTEK 7030”, “IOTEK 7510”, “IOTEK 7520”, “IOTEK 8000”, and “IOTEK 8030”, manufactured by ExxonMobil Chemical Corporation. Two or more ionomer resins may be used in combination.

The resin composition of the mid layer 6 may include a styrene block-containing thermoplastic elastomer. The styrene block-containing thermoplastic elastomer includes a polystyrene block as a hard segment, and a soft segment. A typical soft segment is a diene block. Examples of compounds for the diene block include butadiene, isoprene, 1,3-pentadiene, and 2,3-dimethyl-1,3-butadiene. Butadiene and isoprene are preferable. Two or more compounds may be used in combination.

Examples of styrene block-containing thermoplastic elastomers include styrene-butadiene-styrene block copolymers (SBS), styrene-isoprene-styrene block copolymers (SIS), styrene-isoprene-butadiene-styrene block copolymers (SIBS), hydrogenated SBS, hydrogenated SIS, and hydrogenated SIBS. Examples of hydrogenated SBS include styrene-ethylene-butylene-styrene block copolymers (SEBS). Examples of hydrogenated SIS include styrene-ethylene-propylene-styrene block copolymers (SEPS). Examples of hydrogenated SIBS include styrene-ethylene-ethylene-propylene-styrene block copolymers (SEEPS).

In light of resilience performance of the golf ball 2, the content of the styrene component in the styrene block-containing thermoplastic elastomer is preferably equal to or greater than 10% by weight, more preferably equal to or greater than 12% by weight, and particularly preferably equal to or greater than 15% by weight. In light of feel at impact of the golf ball 2, the content is preferably equal to or less than 50% by weight, more preferably equal to or less than 47% by weight, and particularly preferably equal to or less than 45% by weight.

In the present invention, styrene block-containing thermoplastic elastomers include an alloy of an olefin and one or more members selected from the group consisting of SBS, SIS, SIBS, SEBS, SEPS, and SEEPS. The olefin component in the alloy is presumed to contribute to improvement of compatibility with another base polymer. The alloy can contribute to the resilience performance of the golf ball 2. An olefin having 2 to 10 carbon atoms is preferable. Examples of suitable olefins include ethylene, propylene, butene, and pentene. Ethylene and propylene are particularly preferable.

Specific examples of polymer alloys include trade names “RABALON T3221C”, “RABALON T3339C”, “RABALON SJ4400N”, “RABALON SJ5400N”, “RABALON SJ6400N”, “RABALON SJ7400N”, “RABALON SJ8400N”, “RABALON SJ9400N”, and “RABALON SR04”, manufactured by Mitsubishi Chemical Corporation. Other specific examples of styrene block-containing thermoplastic elastomers include trade name “Epofriend A1010” manufactured by Daicel Chemical Industries, Ltd., and trade name “SEPTON HG-252” manufactured by Kuraray Co., Ltd.

In light of feel at impact, the proportion of the styrene block-containing thermoplastic elastomer to the entire base polymer is preferably equal to or greater than 3% by weight and particularly preferably equal to or greater than 5% by weight. In light of resilience performance, the proportion is preferably equal to or less than 20% by weight, more preferably equal to or less than 15% by weight, and particularly preferably equal to or less than 10% by weight.

The resin composition of the mid layer 6 may include a filler for the purpose of specific gravity adjustment and the like. Examples of suitable fillers include zinc oxide, barium sulfate, calcium carbonate, and magnesium carbonate. The resin composition may include powder of a metal with a high specific gravity. Specific examples of metals with a high specific gravity include tungsten and molybdenum. The amount of the filler is determined as appropriate so that the intended specific gravity of the mid layer 6 is accomplished. The resin composition may include a coloring agent, crosslinked rubber powder, or synthetic resin powder. When the hue of the golf ball 2 is white, a typical coloring agent is titanium dioxide.

The mid layer 6 preferably has a hardness Hm of equal to or greater than 55. With the golf ball 2 that includes the mid layer 6 having a hardness Hm of equal to or greater than 55, the spin rate upon a shot with a driver is suppressed. The mid layer 6 can contribute to the flight performance of the golf ball 2. In this respect, the hardness Hm is more preferably equal to or greater than 58 and particularly preferably equal to or greater than 61. In light of controllability upon hitting with a short iron, the hardness Hm is preferably equal to or less than 80, more preferably equal to or less than 75, and particularly preferably equal to or less than 72.

The hardness Hm of the mid layer 6 is measured according to the standards of “ASTM-D 2240-68”. The hardness Hm is measured with a Shore D type hardness scale mounted to an automated hardness meter (trade name “digi test II” manufactured by Heinrich Bareiss Prüfgeratebau GmbH). For the measurement, a sheet that is formed by hot press, is formed from the same material as that of the mid layer 6, and has a thickness of about 2 mm is used. Prior to the measurement, a sheet is kept at 23° C. for two weeks. At the measurement, three sheets are stacked.

The mid layer 6 has a thickness Tm of preferably equal to or greater than 0.3 mm but equal to or less than 2.5 mm. With the golf ball 2 that includes a mid layer 6 having a thickness Tm of equal to or greater than 0.3 mm, the spin upon a shot with a driver is suppressed. The golf ball 2 that includes the mid layer 6 having a thickness Tm of equal to or greater than 0.3 mm also has excellent durability. In these respects, the thickness Tm is more preferably equal to or greater than 0.5 mm and particularly preferably equal to or greater than 0.8 mm. The golf ball 2 that includes the mid layer 6 having a thickness Tm of equal to or less than 2.5 mm has excellent feel at impact. In this respect, the thickness Tm is more preferably equal to or less than 2.0 mm and particularly preferably equal to or less than 1.8 mm. The thickness Tm is measured at a position immediately below the land 12.

The mid layer 6 preferably has a volume Vm of equal to or greater than 3000 mm³ but equal to or less than 10000 mm³. With the golf ball 2 that includes the mid layer 6 having a volume Vm of equal to or greater than 3000 mm³, spin upon a shot with a driver is suppressed. The golf ball 2 that includes the mid layer 6 having a volume Vm of equal to or greater than 3000 mm³ also has excellent durability. In these respects, the volume Vm is more preferably equal to or greater than 4000 mm³ and particularly preferably equal to or greater than 5000 mm³. The golf ball 2 that includes the mid layer 6 having a volume Vm of equal to or less than 10000 mm³ has excellent feel at impact. In this respect, the volume Vm is more preferably equal to or less than 9000 mm³ and particularly preferably equal to or less than 8500 mm³. When the mid layer 6 is recessed at positions immediately below the dimples 10, the volume Vm is determined on the assumption that these recesses are not present. In the present embodiment, the volume Vm is a value obtained by subtracting the volume of the core 4 from the volume of a sphere having a diameter equal to the diameter of a sphere consisting of the core 4 and the mid layer 6.

The golf ball 2 may include two or more mid layers positioned between the core 4 and the cover 8. In this case, the thickness Tm and the volume Vm of each mid layer preferably fall within the above ranges.

The cover 8 is the outermost layer except the mark layer and the paint layer. The cover 8 is formed from a resin composition. Examples of the base polymer of the resin composition include polyurethanes, ionomer resins, polyesters, polyamides, polyolefins, and polystyrenes. A preferable base polymer in light of controllability upon hitting with a short iron is a polyurethane. When a polyurethane and another resin are used in combination for the cover 8, the proportion of the polyurethane to the entire base resin is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 60% by weight, and particularly preferably equal to or greater than 70% by weight.

The resin composition of the cover 8 may include a thermoplastic polyurethane, or may include a thermosetting polyurethane. In light of productivity, the thermoplastic polyurethane is preferable. The thermoplastic polyurethane includes a polyurethane component as a hard segment, and a polyester component or a polyether component as a soft segment. The thermoplastic polyurethane is flexible. The cover 8 in which the polyurethane is used has excellent scuff resistance.

The thermoplastic polyurethane has a urethane bond within the molecule. The urethane bond can be formed by reacting a polyol with a polyisocyanate. The polyol, as a material for the urethane bond, has a plurality of hydroxyl groups. Low-molecular-weight polyols and high-molecular-weight polyols can be used.

Examples of low-molecular-weight polyols include diols, triols, tetraols, and hexaols. Specific examples of diols include ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propanediol, 1,3-propanediol, 2-methyl-1,3-propanediol, dipropylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 2,3-dimethyl-2,3-butanediol, neopentyl glycol, pentanediol, hexanediol, heptanediol, octanediol, and 1,6-cyclohexanedimethylol. Aniline diols or bisphenol A diols may be used. Specific examples of triols include glycerin, trimethylol propane, and hexanetriol. Specific examples of tetraols include pentaerythritol and sorbitol.

Examples of high-molecular-weight polyols include polyether polyols such as polyoxyethylene glycol (PEG), polyoxypropylene glycol (PPG), and polytetramethylene ether glycol (PTMG); condensed polyester polyols such as polyethylene adipate (PEA), polybutylene adipate (PBA), and polyhexamethylene adipate (PHMA); lactone polyester polyols such as poly-ε-caprolactone (PCL); polycarbonate polyols such as polyhexamethylene carbonate; and acrylic polyols. Two or more polyols may be used in combination. In light of feel at impact of the golf ball 2, the high-molecular-weight polyol has a number average molecular weight of preferably equal to or greater than 400 and more preferably equal to or greater than 1000. The number average molecular weight is preferably equal to or less than 10000.

Examples of polyisocyanates, as a material for the urethane bond, include aromatic diisocyanates, alicyclic diisocyanates, and aliphatic diisocyanates. Two or more types of diisocyanates may be used in combination.

Examples of aromatic diisocyanates include 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 4,4′-diphenylmethane diisocyanate (MDI), 1,5-naphthylene diisocyanate (NDI), 3,3′-bitolylene-4,4′-diisocyanate (TODI), xylylene diisocyanate (XDI), tetramethylxylylene diisocyanate (TMXDI), and paraphenylene diisocyanate (PPDI). One example of aliphatic diisocyanates is hexamethylene diisocyanate (HDI). Examples of alicyclic diisocyanates include 4,4′-dicyclohexylmethane diisocyanate (H₁₂MDI), 1,3-bis(isocyanatomethyl)cyclohexane (H₆XDI), isophorone diisocyanate (IPDI), and trans-1,4-cyclohexane diisocyanate (CHDI). 4,4′-dicyclohexylmethane diisocyanate is preferable.

Specific examples of the thermoplastic polyurethane include trade names “Elastollan NY80A”, “Elastollan NY82A”, “Elastollan NY84A”, “Elastollan NY85A”, “Elastollan NY90A”, “Elastollan NY95A”, “Elastollan NY97A”, “Elastollan NY585”, and “Elastollan KP016N”, manufactured by BASF Japan Ltd.; and trade names “RESAMINE P4585LS” and “RESAMINE PS62490”, manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.

The resin composition of the cover 8 may include a coloring agent, a filler, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like in an adequate amount. When the hue of the golf ball 2 is white, a typical coloring agent is titanium dioxide.

In light of durability of the cover 8, the cover 8 has a Shore D hardness Hc of preferably equal to or greater than 15, more preferably equal to or greater than 18, and particularly preferably equal to or greater than 20. In light of controllability upon hitting with a short iron, the hardness Hc is preferably equal to or less than 40, more preferably equal to or less than 37, and particularly preferably equal to or less than 34.

The hardness Hc of the cover 8 is measured according to the standards of “ASTM-D 2240-68”. The hardness Hc is measured with a Shore D type hardness scale mounted to an automated hardness meter (trade name “digi test II” manufactured by Heinrich Bareiss Prüfgeratebau GmbH). For the measurement, a sheet that is formed by hot press, is formed from the same material as that of the cover 8, and has a thickness of about 2 mm is used. Prior to the measurement, a sheet is kept at 23° C. for two weeks. At the measurement, three sheets are stacked.

In light of controllability upon hitting with a short iron, the cover 8 has a thickness Tc of preferably equal to or greater than 0.1 mm, more preferably equal to or greater than 0.3 mm, and particularly preferably equal to or greater than 0.4 mm. In light of flight performance upon a shot with a driver, the thickness Tc is preferably equal to or less than 2.0 mm, more preferably equal to or less than 1.5 mm, and particularly preferably equal to or less than 1.0 mm. The thickness Tc is measured at a position immediately below the land 12.

The cover 8 preferably has a virtual volume Vc of equal to or greater than 1000 mm³ but equal to or less than 5000 mm³. The golf ball 2 that includes the cover 8 having a volume Vc of equal to or greater than 1000 mm³ has excellent controllability upon hitting with a short iron. In this respect, the volume Vc is more preferably equal to or greater than 1500 mm³ and particularly preferably equal to or greater than 2000 mm³. The golf ball 2 that includes the cover 8 having a volume Vc of equal to or less than 5000 mm³ has excellent flight performance upon a shot with a driver. In this respect, the volume Vc is more preferably equal to or less than 4000 mm³ and particularly preferably equal to or less than 3500 mm³. The volume Vc is determined on the assumption that the dimples 10 are not present. In the present embodiment, the volume Vc is a value obtained by subtracting the volume of a second sphere having a diameter equal to the diameter of the sphere consisting of the core 4 and the mid layer 6, from the volume of a first sphere having a diameter equal to the diameter of a sphere consisting of the core 4, the mid layer 6, and the cover 8.

For forming the cover 8, known methods such as injection molding, compression molding, and the like can be used. When forming the cover 8, the dimples 10 are formed by pimples formed on the cavity face of a mold.

The golf ball 2 may include a reinforcing layer between the mid layer 6 and the cover 8. The reinforcing layer firmly adheres to the mid layer 6 and also to the cover 8. The reinforcing layer suppresses separation of the mid layer 6 from the cover 8. The reinforcing layer is formed from a resin composition. Examples of a preferable base polymer of the reinforcing layer include two-component curing type epoxy resins and two-component curing type urethane resins.

The golf ball 2 has an amount of compressive deformation Sb of preferably equal to or greater than 2.0 mm but equal to or less than 3.5 mm. The golf ball 2 having an amount of compressive deformation of equal to or greater than 2.0 mm has excellent feel at impact. In this respect, the amount of compressive deformation Sb is preferably equal to or greater than 2.2 mm and particularly preferably equal to or greater than 2.3 mm. The golf ball 2 having an amount of compressive deformation Sb of equal to or less than 3.5 mm has excellent flight performance upon a shot with a driver. In this respect, the amount of compressive deformation Sb is more preferably equal to or less than 3.2 mm and particularly preferably equal to or less than 3.0 mm.

For measurement of the amount of compressive deformation, a YAMADA type compression tester is used. In the tester, the golf ball 2 is placed on a hard plate made of metal. Next, a cylinder made of metal gradually descends toward the golf ball 2. The golf ball 2, squeezed between the bottom face of the cylinder and the hard plate, becomes deformed. A migration distance of the cylinder, starting from the state in which an initial load of 98 N is applied to the golf ball 2 up to the state in which a final load of 1274 N is applied thereto, is measured. A moving speed of the cylinder until the initial load is applied is 0.83 mm/s. A moving speed of the cylinder after the initial load is applied until the final load is applied is 1.67 mm/s.

In the golf ball 2, a value V1 calculated by the following mathematical formula exceeds 63.

V1=(DH*Hm)/(Hc*Tc)

In other words, the golf ball 2 meets the following mathematical formula (1.1).

(DH*Hm)/(Hc*Tc)>63  (1.1)

According to the finding by the present inventor, the value V1 correlates with the spin rate upon a shot with a driver. With the golf ball 2 that meets the mathematical formula (1.1), the spin upon a shot with a driver is suppressed. The golf ball 2 has excellent flight performance upon a short with a driver. In this respect, the value V1 is more preferably equal to or greater than 80 and particularly preferably equal to or greater than 95. In light of feel at impact, the value V1 is preferably equal to or less than 180, more preferably equal to or less than 150, and particularly preferably equal to or less than 130.

In the golf ball 2, a value V2 calculated by the following mathematical formula exceeds 3.0 and is less than 7.5.

V2=(Vm*Hm)/(Vc*Hc)

In other words, the golf ball 2 meets the following mathematical formula (1.2).

3.0<((Vm*Hm)/(Vc*Hc))<7.5  (1.2)

The golf ball 2 in which the value V2 exceeds 3.0 has excellent durability. Furthermore, the golf ball 2 in which the value V2 exceeds 3.0 has excellent flight performance upon a shot with a driver. In these respects, the value V2 is more preferably equal to or greater than 3.3 and particularly preferably equal to or greater than 3.5. The golf ball 2 in which the value V2 is less than 7.5 has excellent controllability upon hitting with a short iron. The golf ball 2 has excellent controllability particularly in a state where the golf ball 2 and a clubface are not wet with water. In this respect, the value V2 is more preferably equal to or less than 6.5 and particularly preferably equal to or less than 5.5.

In the golf ball 2 that includes the cover 8 having a low hardness Hc and a small thickness Tc, the mathematical formulas (1.1) and (1.2) can be met.

As shown in FIGS. 2 and 3, the contour of each dimple 10 is circular. The golf ball 2 has dimples A each having a diameter of 4.60 mm; dimples B each having a diameter of 4.50 mm; dimples C each having a diameter of 4.35 mm; dimples D each having a diameter of 4.00 mm; and dimples E each having a diameter of 3.00 mm. The number of types of the dimples 10 is five. The golf ball 2 may have non-circular dimples instead of the circular dimples 10 or together with circular dimples 10.

The number of the dimples A is 24; the number of the dimples B is 12; the number of the dimples C is 252; the number of the dimples D is 24; and the number of the dimples E is 12. The total number of the dimples 10 is 324. A dimple pattern is formed by these dimples 10 and the land 12.

FIG. 4 shows a cross section of the golf ball 2 along a plane passing through the central point of the dimple 10 and the central point of the golf ball 2. In FIG. 4, the top-to-bottom direction is the depth direction of the dimple 10. In FIG. 4, a chain double-dashed line 14 indicates a phantom sphere 14. The surface of the phantom sphere 14 is the surface of the golf ball 2 when it is postulated that no dimple 10 exists. The diameter of the phantom sphere 14 is equal to the diameter of the golf ball 2. The dimple 10 is recessed from the surface of the phantom sphere 14. The land 12 coincides with the surface of the phantom sphere 14. In the present embodiment, the cross-sectional shape of each dimple 10 is substantially a circular arc.

In FIG. 4, an arrow Dm indicates the diameter of the dimple 10. The diameter Dm is the distance between two tangent points Ed appearing on a tangent line Tg that is drawn tangent to the far opposite ends of the dimple 10. Each tangent point Ed is also the edge of the dimple 10. The edge Ed defines the contour of the dimple 10. In FIG. 4, a double ended arrow Dp1 indicates a first depth of the dimple 10. The first depth Dp1 is the distance between the deepest part of the dimple 10 and the surface of the phantom sphere 14. In FIG. 4, a double ended arrow Dp2 indicates a second depth of the dimple 10. The second depth Dp2 is the distance between the deepest part of the dimple 10 and the tangent line Tg.

The diameter Dm of each dimple 10 is preferably equal to or greater than 2.0 mm but equal to or less than 6.0 mm. The dimple 10 having a diameter Dm of equal to or greater than 2.0 mm contributes to flight performance upon a shot with a driver. In this respect, the diameter Dm is more preferably equal to or greater than 2.5 mm and particularly preferably equal to or greater than 2.8 mm. The dimple 10 having a diameter Dm of equal to or less than 6.0 mm does not impair a fundamental feature of the golf ball 2 being substantially a sphere. In this respect, the diameter Dm is more preferably equal to or less than 5.5 mm and particularly preferably equal to or less than 5.0 mm.

In the case of a non-circular dimple, a circular dimple 10 having the same area as that of the non-circular dimple is assumed. The diameter of the assumed dimple 10 can be regarded as the diameter of the non-circular dimple.

The ratio Pd of the diameter Dm of each dimple 10 relative to the diameter of the golf ball 2 is preferably equal to or greater than 9.60% but equal to or less than 10.37%. The dimple 10 having a ratio Pd of equal to or greater than 9.60% contributes to flight performance upon a shot with a driver. In this respect, the ratio Pd is more preferably equal to or greater than 9.90% and particularly preferably equal to or greater than 10.10%. The dimple 10 having a ratio Pd of equal to or less than 10.37% does not impair a fundamental feature of the golf ball 2 being substantially a sphere. In this respect, the ratio Pd is more preferably equal to or less than 10.32% and particularly preferably equal to or less than 10.27%.

The ratio Rs of the number of the dimples 10 each having a ratio Pd of equal to or greater than 9.60% but equal to or less than 10.37%, relative to the total number of the dimples 10, is preferably equal to or greater than 50%. The dimple pattern having a ratio Rs of equal to or greater than 50% contributes to flight performance upon a shot with a driver. In this respect, the ratio Rs is more preferably equal to or greater than 60% and particularly preferably equal to or greater than 70%. The ratio Rs may be 100%.

The ratio Rs′ of the number of the dimples 10 each having a ratio Pd of equal to or greater than 10.10% but equal to or less than 10.37%, relative to the total number of the dimples 10, is preferably equal to or greater than 50%. The dimple pattern having a ratio Rs′ of equal to or greater than 50% contributes to flight performance upon a shot with a driver. In this respect, the ratio Rs′ is more preferably equal to or greater than 60% and particularly preferably equal to or greater than 70%. The ratio Rs′ may be 100%.

The ratio of the number of the dimples 10 each having a ratio Pd exceeding 10.37%, relative to the total number of the dimples 10, is preferably less than 50%. With the dimple pattern of which this ratio is less than 50%, the degree of freedom in designing a dimple pattern is high, and therefore the width of the land 12 is less likely to be excessively large. In this respect, this ratio is more preferably equal to or less than 30% and particularly preferably equal to or less than 10%. This ratio may be zero.

In light of suppression of rising of the golf ball 2 during flight, the first depth Dp1 of each dimple 10 is preferably equal to or greater than 0.10 mm, more preferably equal to or greater than 0.13 mm, and particularly preferably equal to or greater than 0.15 mm. In light of suppression of dropping of the golf ball 2 during flight, the first depth Dp1 is preferably equal to or less than 0.65 mm, more preferably equal to or less than 0.60 mm, and particularly preferably equal to or less than 0.55 mm.

The area S of the dimple 10 is the area of a region surrounded by the contour line of the dimple 10 when the central point of the golf ball 2 is viewed at infinity. In the case of a circular dimple 10, the area S is calculated by the following mathematical formula.

S=(Dm/2)²*π

In the golf ball 2 shown in FIGS. 2 and 3, the area of each dimple A is 16.62 mm²; the area of each dimple B is 15.90 mm²; the area of each dimple C is 14.86 mm²; the area of each dimple D is 12.57 mm²; and the area of each dimple E is 7.07 mm².

In the present invention, the ratio of the sum of the areas S of all the dimples 10 relative to the surface area of the phantom sphere 14 is referred to as an occupation ratio So. In light of flight performance upon a shot with a driver, the occupation ratio So is preferably equal to or greater than 81.0%, more preferably equal to or greater than 82.0%, and particularly preferably equal to or greater than 83.0%. The occupation ratio So is preferably equal to or less than 95%. In the golf ball 2 shown in FIGS. 2 and 3, the total area of the dimples 10 is 4721.1 mm². The surface area of the phantom sphere 14 of the golf ball 2 is 5728.0 mm², so that the occupation ratio So is 82.4%.

From the standpoint that a sufficient occupation ratio is achieved, the total number N of the dimples 10 is preferably equal to or greater than 250, more preferably equal to or greater than 280, and particularly preferably equal to or greater than 300. From the standpoint that each dimple 10 can contribute to turbulization, the total number N of the dimples 10 is preferably equal to or less than 450, more preferably equal to or less than 400, and particularly preferably equal to or less than 380.

In the present invention, the “volume of the dimple” means the volume of a portion surrounded by the surface of the phantom sphere 14 and the surface of the dimple 10. In light of suppression of rising of the golf ball 2 during flight, the total volume of all the dimples 10 is preferably equal to or greater than 450 mm³, more preferably equal to or greater than 480 mm³, and particularly preferably equal to or greater than 500 mm³. In light of suppression of dropping of the golf ball 2 during flight, the total volume is preferably equal to or less than 750 mm³, more preferably equal to or less than 730 mm³, and particularly preferably equal to or less than 710 mm³.

In a graph shown in FIG. 5, the horizontal axis indicates the occupation ratio So of the dimples 10. In this graph, the vertical axis indicates the ratio Rs of the number of the dimples 10 each having a ratio Pd of equal to or greater than 9.60% but equal to or less than 10.37%, relative to the total number of the dimples 10. A straight line indicated by reference sign L1 in this graph is represented by the following mathematical formula.

Rs=−2.5*So+273

The golf ball 2 that belongs to the zone above the straight line L1 in this graph meets the following mathematical formula (2.1).

Rs−2.5*So+273  (2.1)

With the golf ball 2 that meets the mathematical formula (2.1), turbulization is promoted. The golf ball 2 has excellent flight performance upon a shot with a driver.

A straight line indicated by reference sign L2 in the graph of FIG. 5 is represented by the following mathematical formula.

Rs=−2.5*So+278

The golf ball 2 that belongs to the zone above the straight line L2 in this graph meets the following mathematical formula (2.2).

Rs−2.5*So+278  (2.2)

With the golf ball 2 that meets the mathematical formula (2.2), turbulization is promoted. The golf ball 2 has excellent flight performance upon a shot with a driver.

A straight line indicated by reference sign L3 in the graph of FIG. 5 is represented by the following mathematical formula.

Rs=−2.5*So+283

The golf ball 2 that belongs to the zone above the straight line L3 in this graph meets the following mathematical formula (2.3).

Rs−2.5*So+283  (2.3)

With the golf ball 2 that meets the mathematical formula (2.3), turbulization is promoted. The golf ball 2 has excellent flight performance upon a shot with a driver.

In a graph shown in FIG. 6, the horizontal axis indicates the occupation ratio So of the dimples 10. In this graph, the vertical axis indicates the ratio Rs′ of the number of the dimples 10 each having a ratio Pd of equal to or greater than 10.10% but equal to or less than 10.37%, relative to the total number of the dimples 10. A straight line indicated by reference sign L4 in this graph is represented by the following mathematical formula.

Rs′=−2.2*So+245

The golf ball 2 that belongs to the zone above the straight line L4 in this graph meets the following mathematical formula (2.4).

Rs′≧−2.2*So+245  (2.4)

With the golf ball 2 that meets the mathematical formula (2.4), turbulization is promoted. The golf ball 2 has excellent flight performance upon a shot with a driver.

A straight line indicated by reference sign L5 in the graph of FIG. 6 is represented by the following mathematical formula.

Rs′=−2.2*So+252

The golf ball 2 that belongs to the zone above the straight line L5 in this graph meets the following mathematical formula (2.5).

Rs′≧−2.2*So+252  (2.5)

With the golf ball 2 that meets the mathematical formula (2.5), turbulization is promoted. The golf ball 2 has excellent flight performance upon a shot with a driver.

As shown in FIG. 3, the surface of the golf ball 2 (or the phantom sphere 14) can be divided into two hemispheres HE by an equator Eq. Specifically, the surface can be divided into a northern hemisphere NH and a southern hemisphere SH. Each hemisphere HE has a pole P. The pole P corresponds to a deepest point of a mold for the golf ball 2.

FIG. 2 shows the northern hemisphere. The southern hemisphere has a pattern obtained by rotating the dimple pattern in FIG. 2 about the pole P. Line segments S1, S2, and S3 shown in FIG. 2 each extend from the pole P. The angle at the pole P between the line segment S1 and the line segment S2 is 120°. The angle at the pole P between the line segment S2 and the line segment S3 is 120°. The angle at the pole P between the line segment S3 and the line segment S1 is 120°.

Of the surface of the golf ball 2 (or the phantom sphere 14), a zone surrounded by the line segment S1, the line segment S2, and the equator Eq is a first spherical triangle T1. Of the surface of the golf ball 2 (or the phantom sphere 14), a zone surrounded by the line segment S2, the line segment S3, and the equator Eq is a second spherical triangle T2. Of the surface of the golf ball 2 (or the phantom sphere 14), a zone surrounded by the line segment S3, the line segment S1, and the equator Eq is a third spherical triangle T3. Each spherical triangle is a unit. The hemisphere HE can be divided into the three units.

When the dimple pattern of the first spherical triangle T1 is rotated by 120° about a straight line connecting the two poles P, the resultant dimple pattern substantially overlaps the dimple pattern of the second spherical triangle T2. When the dimple pattern of the second spherical triangle T2 is rotated by 120° about the straight line connecting the two poles P, the resultant dimple pattern substantially overlaps the dimple pattern of the third spherical triangle T3. When the dimple pattern of the third spherical triangle T3 is rotated by 120° about the straight line connecting the two poles P, the resultant dimple pattern substantially overlaps the dimple pattern of the first spherical triangle T1. In other words, the dimple pattern of the hemisphere is composed of three units that are rotationally symmetrical to each other.

A pattern obtained by rotating the dimple pattern of each hemisphere HE by 120° about the straight line connecting the two poles P substantially overlaps the dimple pattern that has not been rotated. The dimple pattern of each hemisphere HE has 120° rotational symmetry.

A line segment S4 shown in FIG. 2 extends from the pole P. The angle at the pole P between the line segment S4 and the line segment S1 is 60°. The angle at the pole P between the line segment S4 and the line segment S2 is 60°. The first spherical triangle T1 (unit) can be divided into a small spherical triangle T1 a and a small spherical triangle T1 b by the line segment S4. The spherical triangle T1 a and the spherical triangle T1 b are small units.

A pattern obtained by inverting the dimple pattern of the spherical triangle T1 a with respect to a plane containing the line segment S4 and the straight line connecting both poles P substantially overlaps the dimple pattern of the spherical triangle T1 b. In other words, the dimple pattern of the first spherical triangle T1 is composed of two small units that are mirror-symmetrical to each other.

Similarly, the dimple pattern of the second spherical triangle T2 is also composed of two small units that are mirror-symmetrical to each other. The dimple pattern of the third spherical triangle T3 is also composed of two small units that are mirror-symmetrical to each other. The dimple pattern of the hemisphere HE is composed of the six small units.

According to the finding by the present inventor, with the golf ball 2 of which the dimple pattern of each hemisphere is composed of three units that are rotationally symmetrical to each other by 120° and the dimple pattern of each unit is composed of two small units that are mirror-symmetrical to each other, turbulization is promoted. The golf ball 2 has excellent flight performance upon a shot with a driver.

EXAMPLES Example 1

A rubber composition was obtained by kneading 100 parts by weight of a high-cis polybutadiene (trade name “BR-730”, manufactured by JSR Corporation), 29.5 parts by weight of zinc diacrylate, 12 parts by weight of zinc oxide, an appropriate amount of barium sulfate, 0.1 parts by weight of 2-naphthalenethiol, 0.3 parts by weight of pentabromo diphenyl disulfide, 0.85 parts by weight of dicumyl peroxide, and 2.0 parts by weight of benzoic acid. This rubber composition was placed into a mold including upper and lower mold halves each having a hemispherical cavity, and heated at 150° C. for 20 minutes to obtain a core with a diameter of 39.7 mm. The amount of barium sulfate was adjusted such that a predetermined ball weight is obtained.

A resin composition Ml was obtained by kneading 47 parts by weight of an ionomer resin (the aforementioned “Himilan 1605”), 50 parts by weight of another ionomer resin (the aforementioned “Himilan AM7329”), 3 parts by weight of a styrene block-containing thermoplastic elastomer (the aforementioned “RABALON T3221C”), and 3 parts by weight of titanium dioxide with a twin-screw kneading extruder. The core was covered with the resin composition Ml by injection molding to form a mid layer with a thickness of 1.0 mm.

A paint composition (trade name “POLIN 750LE”, manufactured by SHINTO PAINT CO., LTD.) including a two-component curing type epoxy resin as a base polymer was prepared. The base material liquid of this paint composition includes 30 parts by weight of a bisphenol A type epoxy resin and 70 parts by weight of a solvent. The curing agent liquid of this paint composition includes 40 parts by weight of a modified polyamide amine, 55 parts by weight of a solvent, and 5 parts by weight of titanium dioxide. The weight ratio of the base material liquid to the curing agent liquid is 1/1. This paint composition was applied to the surface of the mid layer with a spray gun, and kept at 23° C. for 12 hours to obtain a reinforcing layer with a thickness of 10 μm.

A resin composition C1 was obtained by kneading 100 parts by weight of a thermoplastic polyurethane elastomer (the aforementioned “Elastollan NY84A”), 0.2 parts by weight of a light stabilizer (trade name “TINUVIN 770”), 4 parts by weight of titanium dioxide, and 0.04 parts by weight of ultramarine blue with a twin-screw kneading extruder. Half shells were obtained from this resin composition by compression molding. The sphere consisting of the core, the mid layer, and the reinforcing layer was covered with two of these half shells. These half shells and the sphere were placed into a final mold that includes upper and lower mold halves each having a hemispherical cavity and having a large number of pimples on its cavity face, and a cover was obtained by compression molding. The thickness of the cover was 0.5 mm. Dimples having a shape that is the inverted shape of the pimples were formed on the cover.

A clear paint including a two-component curing type polyurethane as a base material was applied to this cover to obtain a golf ball of Example 1 with a diameter of about 42.7 mm and a weight of about 45.6 g. Dimple specifications Dl of the golf ball are shown in detail in Tables 5 and 6 below.

Examples 2 to 8 and Comparative Examples 1 to 7

Golf balls of Examples 2 to 8 and Comparative Examples 1 to 7 were obtained in the same manner as Example 1, except the specifications of the core, the mid layer, the cover, and the dimples were as shown in Tables 7 to 9 below. The specifications of the core are shown in detail in Tables 1 and 2 below. The specifications of the mid layer are shown in detail in Table 3 below. The specifications of the cover are shown in detail in Table 4 below. The specifications of the dimples are shown in detail in Tables 5 and 6 below.

[Flight Test]

A driver (trade name “XXIO8”, manufactured by DUNLOP SPORTS CO. LTD., shaft hardness: R, loft angle: 10.5°) was attached to a swing machine manufactured by Golf Laboratories, Inc. A golf ball was hit under a condition of a head speed of 45 m/sec, and the initial speed, the spin rate, and the flight distance of the golf ball were measured. The flight distance is the distance between the point at the hit and the point at which the ball stopped. The average value of data obtained by 12 measurements is shown in Tables 7 to 9 below.

[Controllability]

A sand wedge (trade name “CG15”, manufactured by Cleveland Golf Company, Inc., loft angle: 52°) was attached to a swing machine manufactured by Golf Laboratories, Inc. A golf ball was hit under a condition of a head speed of 21 m/sec, and the spin rate was measured. The measurement was performed in a state where no water was attached to the golf ball and the clubface. The average of values obtained by 12 measurements is shown in Tables 7 to 9 below.

[Durability]

Each golf ball was caused to collide 100 times against a metallic plate at a speed of 45 m/s. Evaluation was performed as follows.

good: among 12 golf balls, there was no broken ball.

bad: among 12 golf balls, there was broken ball(s).

The results are shown in Tables 7 to 9 below.

TABLE 1 Composition of Core (parts by weight) I II III IV V Polybutadiene 100 100 100 100 100 Zinc diacrylate 29.5 22.0 31.3 26.5 25.0 Zinc oxide 12 12 12 12 12 Barium sulfate * * * * * 2-naphthalenethiol 0.1 0.1 0.1 0.1 0.1 Pentabromo diphenyl 0.3 0.3 0.3 0.3 0.3 disulfide Dicumyl peroxide 0.85 0.85 0.85 0.85 0.85 Benzole acid 2.0 2.0 2.0 2.0 2.0 Crosslinking temperature 150 150 150 150 150 (° C.) Crosslinking time period 20 20 20 20 20 (min) * Appropriate amount

TABLE 2 Composition of Core (parts by weight) VI VII VIII IX Polybutadiene 100 100 100 100 Zinc diacrylate 28.5 20.7 22.5 29.5 Zinc oxide 12 12 12 12 Barium sulfate * * * * 2-naphthalenethiol 0.1 0.1 — — Pentabromo diphenyl 0.3 0.3 0.3 0.1 disulfide Dicumyl peroxide 0.85 0.85 0.85 0.85 Benzoic acid 2.0 — — — Crosslinking temperature 150 160 160 140 (° C.) Crosslinking time period 20 20 20 20 (min) * Appropriate amount

TABLE 3 Composition of Mid Layer (parts by weight) M1 M2 M3 M4 Surlyn 8150 — 50 — — Himilan 1605 47 — — 50 Himilan AM7329 50 50 26 50 Himilan 7337 — — 26 — RABALON T3221C  3 — 48 — Titanium dioxide  3  3  3  3 Hardness Hm 63 68 35 65 (Shore D)

TABLE 4 Composition of Cover (parts by weight) C1 C2 C3 C4 Elastollan NY80A — 100 — — Elastollan NY84A 100 — — — Elastollan NY88A — — — 100 Elastollan NY97A — — 100 — TINUVIN 770 0.2 0.2 0.2 0.2 Titanium dioxide 4 4 4 4 Ultramarine blue 0.04 0.04 0.04 0.04 Hardness Hc 31 27 47 36 (Shore D)

TABLE 5 Specifications of Dimples Num- Dm Dp2 Dp1 R Volume Pd ber (mm) (mm) (mm) (mm) (mm³) (%) D1 A 24 4.60 0.135 0.2592 19.66 1.123 10.77 B 12 4.50 0.135 0.2539 18.82 1.075 10.54 C 252 4.35 0.135 0.2461 17.59 1.004 10.19 D 24 4.00 0.135 0.2289 14.88 0.850 9.37 E 12 3.00 0.135 0.1878 8.40 0.478 7.03 D2 A 24 4.60 0.135 0.2592 19.66 1.123 10.77 B 54 4.50 0.135 0.2539 18.82 1.075 10.54 C 210 4.40 0.135 0.2487 17.99 1.028 10.30 D 24 4.00 0.135 0.2289 14.88 0.850 9.37 E 12 3.00 0.135 0.1878 8.40 0.478 7.03 D3 A 24 4.60 0.135 0.2592 19.66 1.123 10.77 B 12 4.50 0.135 0.2539 18.82 1.075 10.54 C 210 4.35 0.135 0.2461 17.59 1.004 10.19 D 66 4.05 0.135 0.2313 15.26 0.871 9.48 E 12 3.00 0.135 0.1878 8.40 0.478 7.03

TABLE 6 Specifications of Dimples D1 D2 D3 Plan view FIG. 2 FIG. 7 FIG. 9 Front view FIG. 3 FIG. 8 FIG. 10 Number 324 324 324 Number of units 3 3 3 Number of small 6 6 6 units So (%) 82.4 84.4 81.1 Rs (%) 77.8 64.8 64.8 Rs + 2.5*So-273 10.80 2.80 −5.45 Mathematical Met Met Unmet formula (1) Rs + 2.5*So-278 5.80 −2.20 −10.45 Mathematical Met Unmet Unmet formula (2) Rs + 2.5*So-283 0.80 −7.20 −15.45 Mathematical Met Unmet Unmet formula (3) Rs′ (%) 77.8 64.8 64.8 Rs′ + 2.2*So-245 14.08 5.48 −1.78 Mathematical Met Met Unmet formula (4) Rs′ + 2.2*So-252 7.08 −1.52 −8.78 Mathematical Met Unmet Unmet formula (5)

TABLE 7 Results of Evaluation Com. Com. Com. Ex. Ex. Ex. Ex. Ex. 1 2 1 2 3 Core Composition I I I I II Diameter (mm) 39.7 39.7 39.7 40.3 37.7 H1 (Shore C) 54 54 54 54 47 H2 (Shore C) 80 80 80 80 73 DH (Shore C) 26 26 26 26 26 Mid layer Composition M1 M1 M1 M2 M2 Hm (Shore D) 63 63 63 68 68 Tm (mm) 1.0 1.0 1.0 0.7 2.0 Volume Vm (mm³) 5205 5205 5205 3697 9911 Cover Composition C1 C1 C1 C1 C1 Hc (Shore D) 31 31 31 31 31 Tc (mm) 0.5 0.5 0.5 0.5 0.5 Virtual volume Vc 2797 2797 2797 2797 2797 (mm³) Dimple D1 D2 D3 D1 D1 Compression Sb (mm) 2.8 2.8 2.8 2.8 2.8 V1 106 106 106 114 114 V2 3.8 3.8 3.8 2.9 7.8 W#1 spin (rpm) 2500 2500 2500 2600 2470 W#1 initial speed 65.5 65.5 65.5 65.7 65.5 (m/s) W#1 flight distance 230.0 230.2 229.2 230.0 230.1 (m) SW spin (rpm) 6550 6550 6550 6550 6300 Durability good good good bad good

TABLE 8 Results of Evaluation Com. Com. Ex. Ex. Ex. Ex. Ex. 4 3 4 5 5 Core Composition III IV V VI III Diameter (mm) 38.5 38.5 38.5 40.1 38.9 H1 (Shore C) 55 51 50 53 55 H2 (Shore C) 81 77 76 79 81 DH (Shore C) 26 26 26 26 26 Mid layer Composition M3 M1 M2 M1 M1 Hm (Shore D) 35 63 68 63 63 Tm (mm) 1.6 1.6 1.6 1.0 1.0 Volume Vm (mm³) 8087 8087 8087 5308 5002 Cover Composition C1 C1 C1 C1 C1 Hc (Shore D) 31 31 31 31 31 Tc (mm) 0.5 0.5 0.5 0.3 0.9 Virtual volume Vc 2797 2797 2797 1694 4941 (mm³) Dimple D1 D1 D1 D1 D1 Compression Sb (mm) 2.8 2.8 2.8 2.8 2.8 V1 59 106 114 176 59 V2 3.3 5.9 6.3 6.4 2.1 W#1 spin (rpm) 2590 2440 2390 2400 2750 W#1 initial speed 65.2 65.4 65.5 65.6 65.3 (m/s) W#1 flight distance 227.8 230.3 230.9 231.4 226.8 (m) SW spin (rpm) 6520 6450 6400 6400 6760 Durability good good good good good

TABLE 9 Results of Evaluation Com. Com. Ex. Ex. Ex. Ex. Ex. 6 6 7 8 7 Core Composition I I VII VIII IX Diameter (mm) 39.7 39.7 39.1 38.5 39.1 H1 (Shore C) 54 54 55 57 60 (Shore C) 80 80 73 75 70 DH (Shore C) 26 26 18 18 10 Mid layer Composition M1 M1 M1 M4 M1 Hm (Shore D) 63 63 63 65 63 Tm (mm) 1.0 1.0 1.3 1.6 1.3 Volume (mm³) 5205 5205 6668 8087 6668 Cover Composition C2 C3 C1 C4 C1 Hc (Shore D) 27 47 31 36 31 Tc (mm) 0.5 0.5 0.5 0.5 0.5 Virtual volume Vc 2797 2797 2797 2797 2797 (mm³) Dimple D1 D1 D1 D1 D1 Compression Sb (mm) 2.8 2.8 3.2 3.2 2.7 V1 121 70 73 65 41 V2 4.3 2.5 4.8 5.2 4.8 W#1 spin (rpm) 2550 2320 2380 2300 2700 W#1 initial speed 65.5 65.5 65.2 65.2 65.7 (m/s) W#1 flight distance 229.5 231.6 229.7 230.5 228.9 (m) SW spin (rpm) 6620 6310 6400 6340 6500 Durability good good good good good

As shown in Tables 7 to 9, the golf ball of each Example has excellent flight performance upon hitting with a driver. Furthermore, the golf ball of each Example has excellent controllability upon hitting with a short iron. From the results of evaluation, advantages of the present invention are clear.

The golf ball according to the present invention is suitable for, for example, playing golf on golf courses and practicing at driving ranges. The above descriptions are merely illustrative examples, and various modifications can be made without departing from the principles of the present invention. 

What is claimed is:
 1. A golf ball comprising a core, a mid layer positioned outside the core, and a cover positioned outside the mid layer, wherein the golf ball has a plurality of dimples on a surface thereof, a ratio So of a sum of areas of the dimples relative to a surface area of a phantom sphere of the golf ball is equal to or greater than 81.0%, a ratio Rs of a number of the dimples each having a diameter of equal to or greater than 9.60% but equal to or less than 10.37%, of a diameter of the golf ball, relative to a total number of the dimples, is equal to or greater than 50%, a dimple pattern of each hemisphere of the golf ball includes three units that are rotationally symmetrical to each other, a dimple pattern of each unit includes two small units that are mirror-symmetrical to each other, and the golf ball meets the following mathematical formula (2.1): Rs≧−2.5*So+273  (2.1).
 2. The golf ball according to claim 1, wherein a difference DH in Shore C hardness between a surface and a central point of the core; a Shore D hardness Hm of the mid layer; and a thickness Tc (mm), and a Shore D hardness Hc of the cover meet the following mathematical formula (1.1): (DH*Hm)/(Hc*Tc)>63  (1.1).
 3. The golf ball according to claim 1, wherein a Shore D hardness Hm, and a volume Vm (mm³) of the mid layer; and a Shore D hardness Hc, and a virtual volume Vc (mm³) of the cover meet the following mathematical formula (1.2): 3.0<((Vm*Hm)/(Vc*Hc))<7.5  (1.2).
 4. The golf ball according to claim 1, wherein a Shore D hardness Hc of the cover is equal to or less than
 40. 5. The golf ball according to claim 1, wherein a Shore D hardness Hm of the mid layer is equal to or greater than
 55. 6. The golf ball according to claim 1, wherein the golf ball meets the following mathematical formula (2.2): Rs≧−2.5*So+278  (2.2).
 7. The golf ball according to claim 6, wherein the golf ball meets the following mathematical formula (2.3): Rs≧−2.5*So+283  (2.3).
 8. The golf ball according to claim 1, wherein a ratio Rs′ of a number of the dimples each having a diameter of equal to or greater than 10.10% but equal to or less than 10.37%, of the diameter of the golf ball, relative to the total number of the dimples, is equal to or greater than 50%, and the golf ball meets the following mathematical formula (2.4): Rs′≧−2.2*So+245  (2.4).
 9. The golf ball according to claim 8, wherein the golf ball meets the following mathematical formula (2.5): Rs′≧−2.2*So+252  (2.5).
 10. The golf ball according to claim 1, wherein a depth of a deepest part of each dimple from a surface of the phantom sphere is equal to or greater than 0.10 mm but equal to or less than 0.65 mm.
 11. The golf ball according to claim 1, wherein a total volume of the dimples is equal to or greater than 450 mm³ but equal to or less than 750 mm³.
 12. The golf ball according to claim 1, wherein a ratio of a number of the dimples each having a diameter exceeding 10.37% of the diameter of the golf ball, relative to the total number of the dimples, is less than 50%. 